Spatial variation in charge risk along the Guyana-Suriname margin

The Guyana-Suriname Basin is a Mesozoic rifted-passive mar­gin along the northeastern coast of South America that has emerged as one of the most prolific deepwater petroleum provinces of the past decade. Since the Liza-1 discovery in 2015, the central play fairway, or “Golden Lane”, has been established by more than 54 discoveries. Production from the Golden Lane has increased to 900,000 bbl/d and is sourced from stacked Upper Albian through Coni­acian – and locally younger – marine source rock acmes. A source rock acme is a specific, geologic time interval com­monly lasting millions of years that marks the preferential deposition and preservation of marine organic matter. Despite the remarkable discovery of the Golden Lane in Guyana, the eastward extension of the play fairway 300 km into offshore Suriname remains uncer­tain, as recent Suriname wells have failed to encounter commercial petroleum.

Source rock potential of the Demerara Plateau

In the first step of charge risk evaluation, we compiled published Rock-Eval pyrol­ysis data from exploration wells on the present shelf and upper slope, including data from five deepwater Demerara Pla­teau sites drilled in 2003 during Leg 207 of the Ocean Drilling Project. We aug­mented this with combined Cenomani­an-Turonian Acme Ultimate Expellable Potential (UEP) values from the Staat­solie Atlas of Suriname (2025).

Multiple source rock acmes in both the Aptian-Lower Albian (‘Aptian’) and Upper Albian, Cenomanian, and Tu­ronian (A3CT) contain Organofacies B kerogen in marine clay-rich mud­stones containing up to 15.8 % organ­ic carbon and with a Hydrogen Index (HI) up to 730 mg HC/g TOC. This translates locally into a truly world-class UEP of up to 126 mmboe/km² for the combined A3CT.

Thermal modelling of the Guyana-Suriname Margin

The Guyana-Suriname margin is charac­terized by along-strike crustal variability with the Golden Lane of Guyana under­lain by the 23-km-thick, non-volcanic, obliquely-rifted margin and the adjacent margin of Suriname underlain by the 25-km-thick and seaward-protruding, Demerara volcanic plateau. This crustal transition straddles the maritime bound­ary between Guyana and Suriname and is expressed by differences in lithospheric thickness, Moho topography, and radio­genic heat production, that combine to produce along-strike variations in source rock thermal stress.

Our full-lithosphere 3D basin model is calibrated using temperature data from seven, widely-spaced exploration wells and therefore provides an example of the value of 3D basin modeling in areas of limited data. Our basin model predicts the spatial distribution of Standard Ther­mal Stress (STS), resulting from variable burial histories and lateral changes in crust-lithosphere heat flow.

STS, volumes and charge fairway

Modeled STS maps for the two source intervals (Figure 2A, Figure 2B) show the oil expulsion window in green is characterized by temperatures >110° C for Organofacies B sources and extends across Blocks 52 and 58 of the western, offshore Suriname area, but does not extend northward of Block 66 in the northwestern area of the seaward-pro­truding, Demerara volcanic plateau. The blue area indicates an area of no expul­sion. The outboard limit of the source rock kitchen only extends 40 km north of the Golden Lane for the A3CT inter­val. Due to deeper burial, the oil expul­sion window at the Aptian level extends 120 km east and north into Blocks 63 and 64 (Figure 2B). Increasing overbur­den and thermal stress levels result in a gas-condensate window (yellow) and dry gas window (red) in this shallower water area, especially near the Guyana- Suriname maritime boundary.

Expelled volumes of petroleum are a function of initial source rock UEP and the level of thermal stress attained. Figure 2C and 2D show expelled oil vol­ume and Gas-Oil Ratio (GOR). In the Golden Lane area, our prediction match­es the observed patterns of low GOR oil fields northwest of Stabroek (e.g. Liza), which become increasingly high GOR with vapor phase accumulations near the maritime boundary, and then become more oil-prone again along the south­eastern Suriname trend (e.g. Grand Mar­gu in Block 58). This variation in GOR is driven by variations in thermal stress that reflect the combined effects of an increase in burial depth and the higher radiogenic heat production from under­lying, more granitic crust.

Note that in parts of Guyana, the model predicts a gassy charge where there are oil accumulations. This is common in many petroleum systems, and concerns the role that the trap plays in determin­ing the GOR. Traps near gas kitchens that accumulate the whole charge history are ‘cumulative’, collecting all the charge over time, whereas a leaky or late-filled trap will behave in a more ‘instantane­ous’ way, preferentially collecting the lat­est fluid expelled from the kitchen. This is likely a major source of the imperfect match (as well as the limited temperature calibration dataset). Mismatches where gas accumulations occur above oil kitch­ens, however, such as at Slonea and Ara­ku, must be explained differently: These accumulations require a deeper source rock as predicted by the Aptian STS map (Figure 2B).

In terms of expelled petroleum volumes, the sweetspot shown by the darkest green area of Figure 2C oc­curs beneath the present outer shelf in Block 52 rather than in the present-day deepwater. This remarkably rich, shel­fal sweetspot has fed updip migration across the hydrostatic shelf and charg­ing of ~1 Bbbl of oil in the coastal, heavy oil fields of Suriname.

Implications for charge and migration risk in offshore Suriname

The highly productive Golden Lane of central and southeastern Guyana is asso­ciated with underlying ACT source beds with STS exceeding 120° C (Figure 2A). Our model shows that the edge of the charge fairway in northwestern Guyana may reflect the decrease in expulsion vol­umes from the distal offshore area, pre­venting the charge front from reaching shallower Cretaceous submarine fan res­ervoirs. This charge risk element is relat­ed to reduction in overburden thickness and/or decreasing RHP on the Jurassic oceanic crust of the deepwater area.

Ongoing and planned drilling of the projected Golden Lane into offshore Suriname includes the Macaw-1 and Araku Deep-1 wells, along with three upcoming wells in Block 52: They will be susceptible to this vertical migration risk. Success in these wells would support our proposed extension of the petrole­um system onto the western flank of the Demerara Plateau and / or the presence of Aptian source rocks in a more expan­sive expulsion kitchen (Figure 2B).

Significance for exploration along the Guyana–Suriname margin

Our model delineates the kitchen limits at the Aptian and A3CT source rock in­tervals over a strike distance of 245 km in offshore Guyana-Suriname. While the A3CT charge fairway terminates within Blocks 52 and 58, we propose a northeastern extension of the Golden Lane kitchen onto the western flank of the Demerara volcanic plateau that is sourced by Lower Albian-Aptian source rocks. Exploration success beyond the Golden Lane of Guyana-Suriname will depend on a combination of sufficient UEP and thermal stress sufficient to overcome vertical migration losses to younger Cretaceous reservoirs.